Distribution Of Energetic Electrons Research Articles

A gyrophase‐bunched electron signature upstream of the Earth's bow shock

A possible sustained signature of a gyrophase‐bunched electron distribution just upstream of the Earth's bow shock is presented. The data, obtained by the ISTP WIND 3‐D Plasma and Energetic Particle Experiment, show several clear examples of a non‐gyrotropic, energetic electron distribution moving into the upstream. The signature is probably that of a locally phase‐trapped electron distribution.

The pitch angle distribution transition of energetic particles at substorm onset observed by GEOS‐2

We discuss here pitch angle distributions of energetic ions and electrons at substorm onset. The transition signatures of ion pitch angle distributions imply that energetic ions experience field‐aligned acceleration after substorm onset which is directed either toward or away from the equator. Our studies indicate that the two often mentioned acceleration mechanisms—parallel electric field and “convection surge” acceleration—cannot explain the observed field‐aligned acceleration signatures. An analysis of the parallel Lorentz equation of ions suggests that under the premise that there exist large temporal variations of the magnetic field, two magnetic field topologies are responsible for the observed field‐aligned acceleration.

Simultaneous triggered VLF emissions and energetic electron distributions observed on POLAR with PWI and HYDRA

We report simultaneous observations of energetic 1–20 keV electrons and VLF emissions triggered within the plasmasphere by pulses from ground based VLF transmitters, using the PWI and HYDRA instruments on the POLAR spacecraft. The 1–20 keV electrons have the correct energy to interact with the input pulses through gyoresonance. Emissions are generated by the pulses only when the particle flux is enhanced well above background and the particle pitch angle distribution is very highly anisotropic, with the average equatorial pitch angle exceeding ∼75°. Because of these high pitch angles, the particles are trapped typically within 7° of the magnetic equator. Only pulses which propagate within whistler mode ducts are observed to trigger emissions. The observed pitch angle anisoptropies are much larger than those assumed in present models of the VLF emission triggering phenomenon, and thus our observations provide a new starting point for understanding the emission process.

Energetic electron butterfly distributions near Io

Pronounced variations in the energetic electron distribution observed by the Energetic Particle Detector during the Galileo flyby of Io are described as a quasi‐adiabatic response to the changing electric and magnetic field environment near the satellite. The energetic particle signatures can therefore be used to remotely sense the spatial distribution of electric and magnetic fields in the vicinity of Io. Electron pitch angle distributions evolve from a normal pancake distribution (peaked at 90° pitch angle) in the undisturbed torus to a butterfly distribution in the strong field depression near Io. The strongest flux depletions at 90° pitch angle result from a reduction in kinetic energy due to conservation of the first adiabatic invariant, as electrons are transported into the vicinity of Io. The magnitude of the flux depletion is related to the spectral index n of the electron energy spectrum (J ∼ E−n). Since the value of n tends to increase with increasing energy, the largest flux drop occurs at higher energy. In the low‐speed wake region downstream of Io, electrons exhibit an abrupt transition to a population which is consistent with trapping on bounce orbits within the magnetic depression near Io. This trapped population, which appears in the same spatial region as intense field‐aligned beams, is not a result of adiabatic transport from a source region upstream of Io. The phase space density of the “trapped” electron population is reduced, compared to the background torus, and particle tracing calculations in a realistic model environment near Io suggest that such electrons must be scattered into the region sampled by Galileo. Torus electrons with energies well above an MeV are excluded from a broad spatial region surrounding Io. This leads to a pronounced drop in the flux of penetrating particles near Io which allows the modest “trapped” electron population to be detected above the background level for energies up to 200 keV.

Time‐dependent modeling of particles and electromagnetic fields during the substorm growth phase: Anisotropy of energetic electrons

We use a bounce averaged drift model with realistic electromagnetic fields together with magnetic field and electron data obtained by CRRES to study energetic electron distributions during the growth phase of an isolated substorm on December 12, 1990. The magnetic field model includes the actual time evolution of the geomagnetic field as measured by CRRES. The inductive electric field caused by the time evolution of the magnetic field configuration is included in the drift model to consider fully electromagnetic fields. The drift motion is computed for all pitch angles and for the entire energy range covered by the medium‐energy spectrometer on CRRES. By using the Liouville theorem we are able to map electron distributions from orbit to orbit to model their time evolution in the model fields. To test the model predictions, we examine the substorm growth phase on December 12, 1990: A quiet period of about 20 hours preceded the growth phase that led to the expansion phase of a 500‐nT substorm. The outer belt energetic electron distributions showed a clear development of magnetic field‐aligned pitch angle anisotropy. This period was covered by two CRRES orbits, 339 and 340. During orbit 339, CRRES measured a quiet time distribution of energetic electrons. During orbit 340 the substorm onset was seen as a rapid dipolarization of the magnetic field and by a dispersionless electron injection. The quiet time fluxes were used as initial conditions for the model for fluxes during the growth phase. We conclude that pitch angle dependent energization of the drifting electrons caused by the inductive electric field plays an essential role in development of the outer belt electron distributions during the substorm growth phase.

Spectral characteristics of waves and particles in the model of cyclotron wave-particle interactions near plasmapause

Abstract. Further analysis of energetic electron precipitation at the evening sector of magnetosphere is performed. In the framework of the quantitative model of cyclotron wave-particle interactions developed in the previous Pasmanik et al. paper, the case of finite spread over energies of initial energetic electron distribution is studied. The solution for distribution function of energetic electron is found. The energetic spectrum of trapped and precipitating electrons and whistler wave spectrum are analysed.Key words. Magnetospheric physics (energetic particles · precipitating; energetic particles · trapped); · Space plasma physics (wave-particle interactions).

A generation mechanism for chorus emission

Abstract. A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2-10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.Key words. Magnetospheric physics (Energetic particles · trapped). Space plasma physics (wave-particle interactions; waves and instabilities)

Cyclotron amplification of whistler waves

Cyclotron wave-particle interactions in the case of well-organized distributions of energetic electrons in an inhomogeneous magnetic field are studied. Step and δ function distributions of field-aligned velocity are considered. The one-hop amplification of whistler waves is calculated analytically and by numerical computation. In rigorous approach, taking into account the third-order term in the spatial dependence of the electron phase with respect to the wave, some new features of the one-hop amplification rate Γ as function of frequency and electron beam parameters are obtained. Γ exhibits a quasi-periodic structure as function of frequency or characteristic electron parallel velocity. For the step-like distribution it remains positive. For the δ-function it changes sign. The dependence of Γ on the total energy, characteristic parallel velocity, position of the injection point in relation to the equator, and dispersion in parallel velocity of energetic electrons such as is discussed.

Energetic auroral electron distributions derived from global X‐ray measurements and comparison with in‐situ particle measurements

On May, 27, 1996, the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on board NASA's POLAR spacecraft was imaging the southern auroral oval during an auroral substorm. Near simultaneous particle measurements by the DMSP F12 and F13 and POLAR satellites allow us to compare measured energetic electron distributions with distributions derived from the x‐ray measurements; agreement is achieved where the assumed electron distribution used in the x‐ray derivations is a reasonable approximation to the measured distribution. The PIXIE data show an energy dispersion in the precipitating electrons in the morning sector such that energy increases with increasing MLT, the result of the dependence of the electron drift speed on energy and its dominance over the loss rate due to precipitation. Strong pitch angle diffusion in the morning sector depletes the source of injected electrons creating the absence of significant electron fluxes, and thus x‐ray fluxes, above 1.5 keV in the afternoon sector.

Cyclotron amplification of whistler waves by electron beams in an inhomogeneous magnetic field

Cyclotron wave‐particle interactions in the case of well‐organized distributions of energetic electrons under an inhomogeneous magnetic field are studied. Step‐like and δ function distributions over the field‐aligned velocity are considered. The one‐hop amplification of whistler waves is calculated by simple analytical solution and numerical computation based on strict approach. The strict consideration, taking into account third‐order expansion of the spatial dependence of the electron phase with respect to the wave, reveals sonic new important features of the one‐hop amplification Γ as a function of frequency and electron beam parameters. The main result is that Γ exhibits a quasi‐periodic structure as a function of wave frequency or characteristic electron parallel velocity, remaining always positive in the case of the step‐like distribution but being sign alternative for δ‐function. Dependence of Γ on the parameters of energetic electrons such as their total energy, characteristic parallel velocity, position of the injection point in relation to the equator, and dispersion in parallel velocity is discussed.

Stereoscopic Observations of Solar Hard X‐Ray Flares Made byUlyssesandYohkoh

The Solar X-Ray/Cosmic Gamma-Ray Burst Experiment aboard the interplanetary spacecraft Ulysses has provided extensive observations of solar hard X-ray flares from a variety of angles with respect to the Sun-Earth line. During the period 1991 October-1993 June, Ulysses observed 13 flares that were also observed by the X-ray instruments aboard the Japanese satellite Yohkoh located near Earth. At least 12 flares were in full view of both the spacecraft. Eight flares, for which hard X-ray spectra were available, are examined to determine the directivity of the 20-125 keV hard X-ray sources in solar flares. They include one flare for which the view angles of Ulysses and Yohkoh were 80° and 25°, respectively. No evidence of systematic directivity was found within the uncertainty (a factor of ~2) of these measurements. These and other observations of directivity at higher energies are consistent with a nearly isotropic distribution of energetic electrons in most solar flares.

Lower ionospheric cyclotron maser theory: A possible source of 2ƒce and 3ƒce auroral radio emissions

A lower ionospheric electron cyclotron maser has recently been suggested as a possible source of natural radio emissions that have been observed with ground‐based instruments in the auroral zone. According to this theory, the observed emissions are interpreted as X mode electromagnetic waves at 2 and/or 3 times the ionospheric electron cyclotron frequency (ƒce). In the present study we carry out a more thorough analysis of the cyclotron maser instability, using a more complete growth rate expression and more realistic energetic electron distribution functions. Growth rates of various electromagnetic modes and harmonics are calculated. It is shown that growth rates for 2ƒce and 3ƒce X modes can exceed collisional ionospheric damping rates, confirming the original estimations. However, the present analysis also reveals that O mode waves at the fundamental cyclotron frequency, ƒce, should also be excited at the source. The propagation of excited electromagnetic waves from the source region to the ground inside a model horizontal density cavity structure is also examined by means of ray tracing technique. It is found that the O mode wave is unable to reach the ground, an explanation of why the emissions at ƒce are not detected on the ground.

Interrelation of noise‐like and discrete ELF/VLF emissions generated by cyclotron interactions

It is shown that quasi‐monochromatic whistler waves (wavelets) can be caused by the strong cyclotron instability, stimulated by hiss emissions. The hiss, generated by the cyclotron instability of an anisotropic smooth energetic electron distribution, creates a step‐like deformation of the distribution function at the boundary between resonant and nonresonant electrons. This deformation leads to the strong amplification of the wavelet whose frequency corresponds to that for cyclotron resonance with electrons at the step. Analytical calculations for this amplification have been made using the rigorous theory of the cyclotron instability in an inhomogeneous magnetic field. The wave amplification can be 2 orders of magnitude greater than that for a smooth distribution function. A self‐consistent computational analysis of the cyclotron instability is developed on the basis of quasi‐linear theory. This confirms both the formation of the step‐like deformation of the distribution function and the wavelet generation.

Long‐term observations of keV ion and electron variability in the outer radiation belt from CRRES

The distribution of energetic electrons and ions with L‐value (L≤8) as measured by the Electron‐Proton Angle Spectrometer onboard CRRES over the whole mission lifetime (July 1990 to October 1991) is presented. The outer radiation belt is shown to be highly dynamic, both in position and in strength. Substantial enhancements of electron and ion flux on short timescales are seen throughout most of the magnetosphere sampled by CRRES, and are strongly related to the Kp index and the ring current as measured by DST. These enhancements decay over less than 30 days. There is evidence of a magnetospheric electron and ion acceleration mechanism of considerable strength and efficiency. Very intense periods of activity (such as the March, 1991 storm) can lead to the creation of an additional, high‐energy belt in the slot region, which persists over a period of many months.

Radiation parameters of an especially great radio microwave burst

By using the data of an especially great solar radio microwave burst occurring at 1700 UT on 30 October, 1992 in a “very active” active region NOAA/USAF Region 7321, on the basis of the theoretical emission mechanism of nonthermal gyro-synchrotron radiation, we have derived the radiation parameters such as the energy distribution of energetic electrons in the source region of the burst, the emission coefficient, absorption coefficient and source function of the gyro-synchrotron radiation and made a simple analysis.

Diagnostics for real‐time plasma control in PBX‐M

An important issue for future tokamaks is real‐time plasma control for the avoidance of magnetohydrodynamic instabilities and other applications that require detailed plasma profile and fluctuation data. Although measurements from diagnostics providing this information require significantly more processing than magnetic flux data, recent advancements could make them practical for adjusting operational settings for plasma heating and current drive systems as well as field coil currents. On the Princeton Beta Experiment‐Modification (PBX‐M), the lower hybrid current drive phasing can be varied during a plasma shot using digitally programmable ferrite phase shifters, and neural beam functions can be fully computer controlled. PBX‐M diagnostics that may be used for control purposes include motional Stark‐effect polarimetry for magnetic field pitch angle profiles, soft x‐ray arrays for plasma position control and the separation of βp from li, hard x‐ray detectors for energetic electron distributions, a multichannel electron cyclotron emission radiometer for ballooning mode identification, and passive plate eddy current monitors for kink stabilization. We will describe the present status of these systems on PBX‐M, and discuss their suitability for feedback applications.

Three-dimensional gyrosynchrotron modelling of radio star spectra: quiescent flux and polarization from the RS CVn binary HR 1099

A three-dimensional gyrosynchrotron model has been developed for radio stars. Systematic procedures have been developed whereby the magnetic field structures and energetic electron distributions can be varied, using physical intuition to model stellar coronae with the prescribed flux and polarization spectra. The Australia Telescope Compact Array has been employed to produce two independent four-frequency continuum radio flux and polarization spectra of HR 1099. The rather flat flux spectrum but changing polarization spectrum of HR 1099 are difficult to reconcile, and impose strong constraints on possible models. Relatively thin shells of radiating electrons in an arcade of magnetic loops reproduce the radio data.

Analytic model for the runaway distribution function in the presence of spatial diffusion

A steady-state kinetic model for runaway electrons in the presence of radial diffusion in a stochastic magnetic field is adopted and solved for a constant magnetic diffusivity. The model is constructed to recover the correct runaway production rate in the absence of spatial diffusion. The parallel energetic electron distribution function f is found by matching the solutions from three regions in parallel velocity space and is employed to form moments of f. Upper and lower bounds on the spatial diffusion are obtained by using these moments and the model exhibits the strong sensitivity to collisionality needed to explain the difference between similar plasmas with little or no hard x-ray signal and those with significant hard x-ray signals.

Electron-beam evolution in partially ionized hydrogenic plasma with return currents

view Abstract Citations (13) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron-Beam Evolution in Partially Ionized Hydrogenic Plasma with Return Currents Syniavskii, D. V. ; Zharkova, V. V. Abstract This paper presents the kinetic equation solution for beam electrons injected during solar flares from the corona to the chromosphere, consisting of hydrogenic plasma with partial ionization. The electrons are considered to lose their energy both in collisional processes with the charged and neutral species of ambient plasma and in ohmic heating by return currents induced in the plasma by beam electrons. The evolution of the energy and angular distributions of energetic electrons is calculated as functions of the column density. The bulk of the electron-beam energy stored in low-energy electrons is shown to be lost in the ambient plasma heating, less via Coulomb collisions at the lower corona and more via ohmic dissipation at the upper chromosphere. More energetic electrons with energies above 120 keV can reach the chromospheric levels with a weak ionized plasma, where a decrease of the Coulomb collisions and the induced electric field of a return current produce beams as well directed as on the top boundary. The X-ray bremsstrahlung polarization is shown to be positive in the range 5%-10%. It is very sensitive to the emergent photon energies below 40 keV and to angles of view for all of the X-ray radiation range. Publication: The Astrophysical Journal Supplement Series Pub Date: February 1994 DOI: 10.1086/191897 Bibcode: 1994ApJS...90..729S Keywords: Beam Injection; Electron Beams; Hydrogen Plasma; Kinetic Equations; Particle Acceleration; Solar Flares; Angular Distribution; Distribution Functions; Energy Dissipation; Fokker-Planck Equation; Plasma Heating; Solar Transition Region; Solar Physics; ACCELERATION OF PARTICLES; PLASMAS; SUN: FLARES; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Electron cyclotron wave generation by relativistic electrons

We show that an energetic electron distribution which has a temperature anisotropy (T⊥b > T∥b), or which is gyrating about a DC magnetic field, can generate electron cyclotron waves with frequencies below the electron cyclotron frequency. Relativistic effects are included in solving the dispersion equation and are shown to be quantitatively important. The basic idea of the mechanism is the coupling of the beam mode to slow waves. The unstable electron cyclotron waves are predominantly electromagnetic and right‐hand polarized. For a low‐density plasma in which the electron plasma frequency is less than the electron cyclotron frequency, the excited waves can have frequencies above or below the electron plasma frequency, depending upon the parameters of the energetic electron distribution. This instability may account for observed Z mode waves in the polar magnetosphere of the Earth and other planets.